The kinetics of production and the properties of the activated T cells which mediate immunity to infection with the facultative, intracellular parasites, Listeria monocytogenes and Mycobacterium tuberculosis will be studied. The relative number of these cells in mouse lymphoid cell populations at any one time during infection will be determined by measuring the capacity of the lymphoid cells to adoptively immunize normal recipients against challenge. The cellular mediators of antimicrobial immunity will be examined in terms of their sensitivity to an antimitotic drug, anti-theta serum, and corticosteroids. Attention will be given to the mechanisms responsible for the rate, magnitude and duration of mediator T cell production. Special attention will be given to examining the properties of those mediator T cells in blood which show a predisposition to enter sterile inflammatory exudates. The mechanism which enables mediator T cells to activate and mobilize macrophages, and thus ensure the expression of antimicrobial immunity, will be studied both in vivo and in vitro. The hypothesis will be tested that the cellular mediators of antimicrobial immunity and the cellular mediators (effectors) of allograft immunity are ontogenetically and functionally related. They will be compared in terms of the kinetics of their production, their sensitivity to corticosteroids, their predilection to enter inflammatory exudates, and their capacity to activate macrophages in vivo.